The present invention relates to a method and apparatus for measuring the distance and/or relative elevation between two points by opto-electronic means. The present apparatus achieves high precision measurements, whereby the topography itself is the only limitation for the distance that may be measured.
U.S. Pat. No. 3,900,260 describes a method and apparatus for measuring the distance and/or a relative elevation between two points, wherein a light source, which may be externally modulated, provides a modulated light beam. The modulated light beam is split into a measuring beam which is reflected by the target and into a reference beam. Both beams are alternately scanned and alternately supplied to a detector by means of an optical light switch such as an electronically controlled galvanometer. The switching or scanning function is subject to a predetermined control function which may be provided by a memory. The output of the light scanner is supplied to a detector and the output of the detector in turn is supplied as one input to a mixing stage which also receives a further frequency signal having a frequency which differs from that of the modulated light beam. The frequency difference is a predetermined value.
In U.S. Pat. No. 3,900,260 a third frequency is produced by a third oscillator, the output of which is supplied to a second mixing stage which also receives the first frequency signal. The third frequency differs from the first and second frequency by a predetermined value. Thus, in the reference two separate frequency channels are established and the difference frequency between the two channels is supplied either to a differentiating circuit or to a logic signal combining circuit to detect the spacing between two pulse flanks defining a time difference which is thus derived from the two frequency channels. The result of this differentiation or signal combination is supplied to a signal evaluation computer which provides a direct display of the measured distance. The operation is such that transit times and transit time variations inherent in the apparatus are eliminated exactly during the measuring which is accomplished on the basis of the precise frequencies of the two frequency channels.
Further, in U.S. Pat. No. 3,900,260 two distances are measured from a measuring point to two points, one of which is arranged vertically above the other with a given spacing. The light beam is reflected from these two points to a target by retroreflectors, whereby the known apparatus is capable of automatically providing a measure for the elevation as well as for the distance along a base line between the measuring point and the target.
U.S. Pat. No. 3,652,161 discloses a method and arrangement for optically measuring distances with high precision by detecting the time of travel of a pulse whereby two separate channels are employed requiring two separate light detectors. One channel processes the reference beam and the other channel continuously processes the reflecting beam. In U.S. Pat. No. 3,642,161 (Ross) a coarse measurement is made by measuring the transit time of a single impulse. A second measurement employs a higher frequency signal and a phase difference between the reference signal and the reflected signal is measured, whereby an ambiguity in the number of wave lengths between the direct and reflected wave is resolved by the coarse measurement.
U.S. Pat. No. 3,765,768 measures the distance and radial speed of a moving object by making use of the so-called "Doppler" frequency of the reflected pulse.
The use of two different channels as in U.S. Pat. No. 3,652,161 and U.S. Pat. No. 3,900,260 is subject to the limitation that different aging and hence different drifting phenomena take place in the two different channels. Such aging and drifting phenomena may adversely influence the precision of the resulting measurements.
Prior art devices also leave room for improvement with regard to the resolution or degree of accuracy in the measurement especially as selecting substantially any desired degree of resolution is concerned.